The present invention relates to methods of and apparatus for the evaluation of corrosion protection afforded to a metallic surface by a protective surface coating thereon.
The present invention relates more specifically to a method of and apparatus for the measurement of the extent of cathodic disbondment of an anti-corrosion protective surface layer overlaying a metallic surface, such as in a metal pipeline, or the like.
There exists at the present time a need for a method of and an apparatus for the non-destructive measurement of the extent of cathodic disbondment of an adherent anti-corrosion protective coating from its associated pipeline outer surface.
Metallic surfaces are adversely affected by numerous corrosive electrolytic fluids that contact these surfaces. In the natural gas and petroleum industries, e.g., corrosion occurs extensively on the outer surface of both implanted and above-ground pipelines.
In order to reduce, or entirely eliminate this undesirable metallic surface corrosion, anti-corrosion protective coatings are extensively used in the pipeline industry. These ubiquitous anti-corrosion protective coatings frequently take the form of a helically-applied tape-like protective outerwrapping. The tape-like protective component may be applied directly over an unprepared pipeline outer surface, or may, in fact, be overlaid onto a primer-coated, pretreated pipeline outer surface.
An important measurable parameter directly relating to the performance of anti-corrosion pipeline protective coatings is that of cathodic disbondment. This property is defined as the extent to which an anti-corrosion protective coating overlaying a metallic surface will disbond as a result of a cathodic reaction, around an unintentionally-induced holiday, or discontinuity, in the protective coating, in a case where the pipe has been subjected to cathodic protection potentials in the soil environment.
Cathodic protection as it is used here, refers to the phenomenon of applying a small potential to a metallic pipeline that is buried in the ground. This imparted cathodic status of the buried pipeline will tend to limit or protect against corrosion attacking the metal surface.
The prior art methods that have been used heretofore by the industry to measure the property of cathodic disbondment is the ASTM/G-8 ("Cathodic Disbonding of Pipeline Coatings") in the United States, and DIN 30-670, a similar method employed throughout Europe.
The prior art methods describe accelerated procedures for the determination of the cathodically disbonded area by means of exposure of the test pipe segment with its adherent anti-corrosion protective coating to a salt electrolyte solution, for a period of from 30 to 90 days, following the cutting of the protective coating in the form of an intentionally-induced holiday, and with a potential being applied to the system.
Following the testing period, the anti-corrosion protective coating is then cut at the intentionally-induced holiday, carefully peeled back from the induced holiday, until resistance is felt, and the extent of the disbonded area is then physically measured.
Some of the major disadvantages of the existing prior art cathodic disbondment measuring methods are the following:
The methods are physically destructive. PA0 They are subjective in interpretion of results, due to subjectively determining the point at which resistance is met. PA0 They are quite time-consuming, requiring longer time periods to complete each test. PA0 The present method is non-destructive, the electrical measurements being performed in situ. PA0 The instant method utilizes recordable electrical measurements, and therefore, is not biased by the subjective interpretations of the operator that are required in the prior art methods. PA0 Information relating to the extent of the cathodically disbonded area, may be obtained on a daily basis, rather than at the conclusion of the prior art's 30 to 90 day testing periods. PA0 The electrical measurements of the present invention are readily amenable to computer storage, manipulation and retrieval of test data. PA0 Finally, the present invention method requires the use of considerably smaller amounts of pipe material than in the prior art method.
The present invention has elegantly circumvented the above-described disadvantages of the prior art methods, and some of its important features are the following: